This invention relates generally to joints in concrete slabs, and more particularly to an improved joint and method of installation to prevent concrete surface deterioration caused by spalling at edges of the joint spaces.
Concrete floor slabs having exposed surfaces subjected to repeated impact loads, such as those produced by hard wheel tires on industrial lift trucks, are susceptible to localized failure at unprotected edges of cracks and joint spaces because of the inherent brittleness and weakness of concrete in both tension and shear. The breakage and crushing type failure at the unprotected edges is generally referred to in the art as "spalling". To reduce the likelihood of edge spalling, joint spaces and cracks are routinely filled with sealant materials in an effort to avoid edge exposure. In today's market, various liquid plastics including epoxies, urethanes and polysulfides are available as joint fillers. Nevertheless, floor joints and cracks in concrete surfaces subjected to hard-wheeled traffic continue to eventually break down because of spalling, regardless of the joint or crack filler material utilized.
Concrete slab shrinkage is a well known ongoing process because of hydration and drying within the concrete mass, and is manifested by steady growth in the width of joint spaces and cracks. The filler material selected must therefore accommodate such long-term slab shrinkage by virtue of its elastic and adhesive bonding properties. While the stresses induced by slab shrinkage are resisted both in the body of certain rigid types of filler materials and at their bonding interfaces with the concrete, eventually the tensile strength of adjacent layers of concrete is exceeded to cause adjacent layer fracture or "re-cracking". Such re-cracking phenomenon creates the very same condition the filler was intended to prevent or repair, i.e., concrete edge exposure. In an attempt to avoid re-cracking failure resulting from induced stresses, a semi-rigid, low-adhesive type of filler material has been formulated, wherein the concrete bonding interfaces of the filler are adhesively weaker than the tensile strength of the filler or the concrete alone, so as to preclude re-cracking of the concrete in spaced adjacency to the filler, as aforementioned. However, filler separation or fracture at the concrete bonding interfaces then occurs in response to shrinkage induced stress resulting in edge exposure and spalling under repeated impact loading.
Various joint filler modifications other than changes in material formulation have been proposed in an effort to deal with the foregoing spalling problem, including the use of plastic divider strips in an enlarged spalling repair patch, or insert elements embedded in the filler during joint installation. For example, a filler body is held compressed by an insert element during joint installation, for subsequent expansion within the joint space according to U.S. Pat. Nos. 3,276,334 and 3,255,680 to Rhodes and Cooper et al, respectively. According to U.S. Pat. No. 4,699,540 to Gibbon, a preformed cylindrical insert is utilized to relieve any strain at the concrete bonding interfaces of the filler caused by concrete expansion. However, none of the foregoing joint filler modifications provides a completely reliable solution to the problem of eventual failure by spalling at filled joint spaces and cracks, related to the aforementioned re-cracking phenomenon caused by long term slab shrinkage.